When radicals of a raw material gas are generated, and a thin film of the raw material is formed on an object of interest, or an object of interest is etched by means of the thus-generated radicals, precise control of such a process requires determination of the atomic particle (e.g., radical) density of a plasma atmosphere for controlling formation of a plasma. For this purpose, such a plasma atmosphere is irradiated with light, and the atomic particle density of the plasma atmosphere is determined on the basis of light absorption features of the particles.
Determination of such a particle density requires a light source. For example, in a method disclosed in Patent Document 1, a light source is provided outside a reaction chamber. A plasma atmosphere is irradiated with light generated by electric discharge of the same gas as the plasma-forming gas from the outside of the reaction chamber, and the light is caused to pass through the plasma atmosphere. The light absorbed by atomic radicals is spectroscopically analyzed by means of a spectrometer provided outside the reaction chamber.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2000-123996